The invention relates to a method for installing optical fibres or cables, using a fluid under pressure, in a tubular section comprising a supply tube and an installation tube, each having an input and an output, the output of the supply tube being connected to the input of the installation tube, a fluid under pressure being fed near the input of the supply tube, and the cable being conducted into the input of the supply tube, and being propelled through the tubular section by the entraining force of the fluid.
Such a method is disclosed, e.g., in EP-A-0 108 590. In this method, an optical cable is conducted into the tube by way of mechanical force, particularly by way of a pair of pressure rolls. To prevent loss of pressure, the input of the tube or of the device is provided, by way of pressure rolls, with a sealing preventing the escape of fluid along the cable or fibre. The pressure rolls serve to overcome the pressure difference between the environment and the space where the fluid pressure is prevailing upon introduction of the cable. In EP-A-0 292 037, a similar method is disclosed, the pressure rolls exercising a force on the cable which exceeds the one required to overcome the pressure difference. With the additional pushing power, a greater installation length may be achieved than with only the entraining force of a fluid, particularly in the event of thicker cables.
For overcoming the force required to introduce the cable into the space where the fluid pressure is present, it is also known to make use of a fluid flowing with increased velocity over a first part, viewed from the input, of the tube length. Exercising mechanical forces, such as in the event of thin or vulnerable fibres or cables, is undesirable on account of the possibility of mechanical damage.
In EP-A-0 345 043 it is disclosed to feed the pressure to the leg of a T-shaped coupling piece, the ongoing portion of the T receiving the cable at one end, and at the other end being connected to a supply tube extending over a relatively modest length, e.g., 1 m, into the installation tube and having a diameter which is 0.1 to 0.8 times the one of [sic] the diameter of the installation tube. In the narrow supply tube, the velocity of the fluid flow is a great deal higher than in the installation tube, as a result of which the required tensile force is generated. A drawback of said solution is that a relatively large portion of the total pressure difference between the input and the output of the installation tube acts on the narrow supply tube, so that either the presssure must be chosen very high for sufficient residual pressure to remain for the actual installation, or the installation length is restricted. The first case often encounters practical, safety and cost problems, and the second is always undesirable, unless the installation is effected over a modest length only.
In EP-A-0 287 225 a method is disclosed, an additional flow rate of the fluid over an initial portion of the installation tube being realised as well. Here, the diameter of the installation tube is the same as the one of the supply tube, and the latter is in fact part of the installation tube. The additional fluid flow is realised by, at the end of the supply tube, where it changes into the installation tube, blowing off part of the fluid by way of an adjustable valve. Said solution has the problem that the velocity of the fluid in the supply tube soon achieves the velocity of sound. In the event of air as a fluid, and a supply tube having a length of several meters, such may already be the case. When achieving the velocity of sound, the energy of the fluid is no longer converted into entraining forces on the cable but into acceleration of the fluid itself.
An additional problem, larger in practise, is that a relatively thin cable in a relatively wide supply tube with respect to said cable, in the event of a high fluid velocity will soon start to twist, so-called buckling, as a result of which the cable will prematurely stick in the tube and can no longer be propelled by the fluid. In the event of narrow supply tubes, such drawback does not exist, but the drawbacks mentioned earlier do.
The object of the invention is to provide for a method enabling the installation of an optical fibre or cable using a fluid, a fluid flow also being applicable for overcoming the pressure difference between the space outside the installation tube and the inside thereof, the fluid flow, over a first portion of the tubular section, having a higher velocity than over the remaining portion of the installation section, without the problems described above occurring.
For this purpose, the invention provides for a method of the aforementioned kind, characterised in that, at the output of the supply tube, at least part of the fluid from the supply tube is discharged, and that, at the input of the installation tube, a second fluid is fed under pressure.
The invention also provides for a device for, using a fluid under pressure, installing optical fibres or cables in a tubular section comprising a supply tube and an installation tube, each having an input and an output, the output of the supply tube being in connection with the input of the installation tube, means being provided for, near the input of the supply tube, feeding a fluid under pressure, and for means to conduct the cable into the input of the supply tube, the cable being propelled through the tubular section by the entraining force of the fluid, characterised in that, at the output of the supply tube, means are provided to discharge at least part of the fluid from the supply tube, and that means are provided for feeding, at the input of the installation tube, a second fluid under pressure.
Essentially, the fluid is preferably completely discharged from the supply tube. In addition, the fluids for the supply tube and the output tube preferably come from one and the same source.